The technique of charging inorganic particles such as silica, alumina, mica and talc in a polysiloxane-based material in order to improve the heat resistance of said material is widely used, but as the charged quantity of inorganic particles increases, heat resistance improves, but the processability and fluidity of said material deteriorates. Therefore, in order to ensure compatibility between properties of heat resistance and processability, Japanese Unexamined Patent Application Publication No. 2003-137627 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-503467, for example, disclose controlling the shape and size of a charged inorganic material and blending spherical particles. In addition, Japanese Unexamined Patent Application Publication Nos. 2007-332104, 2008-19426 and 2008-38137, for example, disclose a method of treating the surface of inorganic particles with an appropriate material and dispersing in a polysiloxane at a high concentration. However, materials obtained using these techniques are not hard and are not suitable for use in high temperatures due to exhibiting high thermal expansion.
The era of extensive commercialization of power semiconductors such as SiC or GaN is approaching. The key to this commercialization is highly heat resistant sealing materials, but there is a need for materials which can withstand continuous use at temperatures of 200° C. or higher, do not undergo peeling from a substrate as a result of thermal history and exhibit low thermal expansion, which is expressed by the coefficient of linear expansion.
As a polysiloxane-based material that exhibits low thermal expansion, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-503142, for example, discloses a novel curable silicone resin. A cured product of this silicone resin exhibits lower thermal expansion than a cured product of a conventional silicone resin, but has a coefficient of linear expansion of approximately 100 ppm in the temperature range of room temperature to 100° C., and therefore still exhibits high thermal expansion. In addition, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-503142 merely discloses the coefficient of linear expansion up to a temperature of approximately 200° C.
Charging inorganic particles is important in order to maintain low thermal expansion in the high temperature region of 250° C. or higher. Japanese Unexamined Patent Application Publication No. H05-105814 discloses a composition obtained by blending spherical inorganic particles in a curable polysiloxane, but this composition becomes gel-like following curing, and therefore exhibits inadequate heat resistance and has a high coefficient of linear expansion. In addition, domestic re-publication No. 2003-29353 discloses a low thermal expansion material comprising an epoxy group-containing polysiloxane and spherical inorganic particles, but this material is a composition in which fluidity is ensured by using a solvent, and is therefore not suitable for use as a sealing material.
In addition, solid epoxy resins able to be transfer molded are well-known (for example, see Japanese Unexamined Patent Application Publication No. H08-311159), but because a highly viscous fluid is injected into a mold at high pressure, this method is not suitable for forming a compact shape or sealing an electronic component, many articles have low glass transition temperatures, and it cannot be said that the coefficient of linear expansion is satisfactory at temperatures higher than this.
Therefore, it is currently the case that there have been no reports of polysiloxane compositions which exhibit good processability and which give hard materials having extremely low coefficients of linear expansion following curing, or of cured products thereof having low thermal expansion.